1. Field of the Invention
The present invention relates to a graphite composite which is usable, for example, in electrode materials, adsorbent materials, lightweight high-conductive materials, superconductive materials, catalysts for polymerization, and catalysts for ammonia synthesis and diamond synthesis.
2. Description of the Related Art
Graphite composites have been proposed, which comprise graphite and fine particles of an element capable of alloying with Li, said particles being finely dispersed in graphite. Such graphite composites are known to be formed through CVD, in which a substance containing an element capable of alloying with Li, such as SiCl.sub.4, is heated to give a gaseous precursor, and this is reacted with graphite at high temperatures of up to about 1000.degree. C. through gaseous reaction to produce a graphite composite.
On the other hand, known are graphite composites comprising graphite particles and particles of a metallic or non-metallic element and containing a complex as formed through intercalation of a metal into layers of graphite crystallites (hereinafter referred to as an intercalation compound). As the means of producing such intercalation compound-containing graphite composites, known are a method for forming intercalation compounds, which comprises heating a metal followed by catalytically reacting the resulting gaseous phase of said metal with graphite (see Japanese Patent Application Laid-Open No. 62-87407), and a method for forming intercalation compounds, which comprises electrochemically intercalating a metal into graphite layers in an electric field (see Japanese Patent Application Laid-Open No. 4-79153). In Japanese Patent Application Laid-Open No. 4-202006, used is a two-bulb method for forming graphite intercalation compounds.
Lithium secondary batteries are known, of which the negative electrode comprises graphite and in which lithium ions reversibly move between the cathode and the negative electrode via an electrolytic solution to produce charging-discharging cycles. As the graphite for such use, employed is powdery graphite to be formed by powdering natural graphite in an mortar into fine particles having a mean particle size of 100 .mu.m or smaller (see Japanese Patent Application Laid-Open No. 6-223821), or synthetic graphite to be synthesized by calcining resins such as synthetic resins having benzene rings, or organic compounds such as cokes and pitch, at temperatures of 1200.degree. C. or lower.
However, the above-mentioned means of powdering natural graphite in a mortar is disadvantageous in that its powdering power is too weak to successfully obtain fine graphite crystallites having particle sizes of not larger than 10 nm. Therefore, if the powdery graphite as produced according to said means is used as the negative electrode material in lithium secondary batteries, the amount of lithium ions to be intercalated into the negative electrode is small and, as a result, the discharging capacity of the batteries is small. On the other hand, fine crystallites of synthetic graphite to be obtained by calcining organic compounds contains large amounts of impurities such as hydrogen and oxygen. Therefore, if such synthetic graphite is used as the negative electrode material in lithium secondary batteries, these impurities disadvantageously react with lithium ions thereby increasing the irreversible capacity of the batteries.
The above-mentioned graphite composite formed through CVD and comprising graphite particles and fine particles of an element capable of alloying with Li is also disadvantageous in that the crystallinity of the graphite particles is poor and some graphite particles may form carbides. Therefore, if the graphite composite of this type is used as the negative electrode material in lithium secondary batteries, such is disadvantageous in that the lithium-discharging capacity of the graphite particles is small at low potential. In addition, the graphite composite formed through CVD contains large amounts of impurities such as hydrogen and oxygen. Therefore, when this is used as the negative electrode material in lithium secondary batteries, the impurities react with lithium thereby increasing the irreversible capacity of the batteries. Moreover, the CVD method itself is expensive, resulting in the increase in the production costs of the graphite composite, and the industrial applicability of the graphite composite is low.
The other graphite composites mentioned hereinabove, which are produced through vapor-phase catalytic reaction of a heated metal vapor with graphite or produced through electrochemical reaction are also disadvantageous in that the intercalation compound and the metal are not satisfactorily finely dispersed in graphite. Therefore, if such graphite composites are used as the negative electrode material in lithium secondary batteries, the metal is dropped off during repeated charging-discharging cycles, resulting in that the graphite composites used could not be active enough as the active material in negative electrodes.